Cause still unknown.
Back in June, a Tesla Model S caught fire while, seemingly, just driving down the street minding its own business. There have been a few other fires involving the premium electric sedan in the past, but those typically involved some sort of collision — usually a very violent one in which the battery pack's integrity was compromised. This one, though, was different and it raised some eyebrows.
Chelsea Handler, who is apparently a celebrity, tweeted (embedded below) out a warning that early owners should beware, saying she would not drive hers again (she later said she traded her car in). Chevrolet, sensing the opportunity to do a good deed/get some press, offered the owner of the Car-BQ a loaner Bolt. Finally, the National Transportation Safety Board announced they were sending a specialist over to observe the automaker's examination of the vehicle, though at the time also said it wasn't launching a formal investigation into the situation. This has changed and they have now released a preliminary summary of the incident.
— Chelsea Handler (@chelseahandler) June 16, 2018
The report details the sequence of events, from the time the driver of the Model S was flagged down until it was taken to a Tesla facility, had its battery removed, drained of charge, then reassembled. It also states that, "The NTSB was provided with information from the initial inspection and participated in the subsequent inspection."
As it is a preliminary report, the NTSB is careful to point out that the information is "subject to change and may contain errors." If you were hoping for the satisfaction of learning the probable cause, you will have to wait a bit longer. The agency goes on to say, "It will be supplemented or corrected during the course of the investigation. No determination of probable cause is made in this report."